Vehicles represented by cars are each provided with seatbelts to protect occupants from unpredictable accidents such as a sudden stop or collision. As such seatbelts, three-point seatbelts that are easy to fit and that exert a relatively high restraining force are commonly used.
The three-point seatbelts are provided with a retractor that winds up a belt to restrain an occupant. This retractor may be provided with a motor for adjusting the tension of the belt in accordance with the force that applies to the occupant. The motor is mechanically connected to a spool for winding up the belt when adjusting the tension of the belt. When there is no need to restrain the occupant such as when the vehicle has been stopped, the motor is disconnected from the spool.
In general, the motor and the spool are connected mechanically through a plurality of gears and a clutch. When the motor and the spool are connected, or when this connection is released, the motor is driven by a relatively strong force. This may cause the occupant to have an uncomfortable feeling due to the sound produced from the motor or the operating sound of the clutch.
So, various techniques have been proposed to reduce the sound produced from the clutch operation.
The device described in Unexamined Japanese Patent Application Kokai Publication No. 2008-30668 supplies a motor's maximum allowable current to the motor when disconnecting the motor and a spool by operating a clutch. This shortens the time required for the clutch operation, and results in reducing the time the clutch operating sound is produced.
The device described in Unexamined Japanese Patent Application Kokai Publication No. 2005-28970 operates a clutch by supplying to a motor a current of a smaller value than the value of the current that is supplied to the motor to rotate a spool. By this means, when operating the clutch, the number of revolutions of the motor is prevented from increasing significantly, which results in reducing the clutch operation sound.
Motors generally require great power at start-up, but, once started, motors do not require much power. So, like the devices described above, when the value of the current that flows in the motor to operate the clutch is kept constant, the number of revolutions of the motor may increase unnecessarily. In this case, a relatively large sound is continuously produced from the motor.
The present invention has been made in view of the above-described backgrounds, and it is therefore an object of the present invention to operate a clutch accurately and reduce the sound produced from the clutch operation.